1. Field of the Invention
The present invention relates to a membrane electrode assembly applied to an electricity generating cell of a fuel cell using a polymer electrolyte membrane, and to a fuel cell using same.
2. Description of the Related Art
In recent years, fuel cells that generate electricity by electrochemical reactions between hydrogen and oxygen have become the subject of interest as an energy supply source. In a fuel cell, for example, using an ion exchange resin membrane, the configuration is generally one in which the ion exchange resin membrane made of a polymer electrolyte is sandwiched between an anode electrode and a cathode electrode. Each of these electrodes is constructed so as to be provided with a catalyst layer that supports an electrochemical reaction and a diffusion layer that functions as an electrical collector.
The catalyst layer is generally formed by dispersing carbon (C) powder that carries a catalyst metal such as platinum (Pt) based metal or the like for the electrode reaction, into an appropriate organic solvent, and forming this into a paste by adding to this an electrolyte solvent, and then providing this on an ion exchange resin membrane, which is a proton-conductive substance. In this fuel cell electrode, therefore, the structure is a gas diffusion type which is based mainly on carbon black, and in which a catalyst metal that supports the electrode reaction and a proton-conductive substance and the like are hardened thereonto. (For example, refer to the non-patent publication, “Development and Application of Solid Polymer Type Fuel Cells” NTS, Inc., page 6).
The electrode reaction occurs at the part at which the electrode and the proton-conductive substance make contact. That is, the reaction occurs with the supply of the reactant gas to the three phase boundary, and because, as shown in FIG. 4, the design of the three phase boundary is extremely difficult art, in the conventional art there was the problem of the insufficient supply of reactant gas to reach the three phase boundary, and a lack of effective utilization of the costly Noble metal catalyst supported in the region of the three phase boundary. For this reason, even if an attempt is made to reduce the amount of Noble metal that is used, this leads to a decrease in the current supply, and it is difficult to reduce the amount of use of the Noble metal without sacrificing electrical generation efficiency. The white circles in FIG. 4 indicate platinum particles which are effective in the reaction, and the black circles in FIG. 4 indicate platinum particles which are not effectively operating in the reaction.
A diffusion layer is generally provided at the above-noted electrode, in which case a diffusion layer is provided between a separator for the purpose of forming a flow passage for the supply and removal of fuel and the catalyst layer, so that the structure is a laminate of separator/diffusion layer/catalyst layer/polymer electrolyte membrane. Because it is particularly necessary to prevent a worsening of the contact resistance between the diffusion layer and catalyst layer and between the separator and the diffusion layer from reducing the electrical generation efficiency, it is essential that surface pressure be applied by tightening with bolts or the like, and there has been the problem of a worsening of membrane durability. Particularly serious problems have been that of reducing the electrical contact resistance between the diffusion layer and catalyst layer and that of improving the diffusion of fuel and the like, that is, of achieving conductivity (for example, refer to Japanese Patent Application Publication No. JP-A-2002-246034).
A reducing of the contact resistance between the catalyst layer and the gas diffusion layer is described in, for example, the Japanese Patent Application Publication JP-A-2001-6699. However, because the constitution is one in which a layer of only carbon particles is provided as a diffusion layer on the catalyst electrode, although there is a small reduction in the internal resistance, the gas diffusion is not improved, and this is insufficient from the standpoint of improving the electrical generation efficiency.
Further, an electrode catalyst using a fibrous carbon as a carrier is described in, for example, the Japanese Patent Application Publication No. JP-A-8-17440. However, with a constitution in which the carbon is merely made fibrous, this is similar to the above-noted related art in that the electrical generation efficiency is insufficient, and in that a detrimental effect on the membrane durability is not avoided.
As described above, although reducing the internal resistance in a fuel cell is effective in increasing the electrical generation efficiency of the fuel cell, the effectiveness of merely adding a diffusion layer using carbon paper or the like in improving the contact resistance is poor. Also, related art has not been established for effectively utilizing, in the electrode reaction, the Noble metal catalyst, which is carried in the region of the three phase boundary that is at a distance from the supply side of the fuel and oxidant gas. That is, the current situation with related art is that it is not yet possible to provide a membrane electrode assembly capable of achieving good electrical generation efficiency without applying high surface pressure by tightening, which adversely affects the membrane.